Recently, electric apparatuses, electronic apparatuses, optical apparatuses, and like products provide higher performance, have smaller sizes, and are more portable, and thus there is a tendency toward an increase in the amount of heat generated inside the products. For example, in the case of household electric appliances, with higher density packaging of printed circuit boards and faster microprocessors, there is a significant increase of power consumption per component, and accordingly there is an increase of the amount of generated heat as well. Therefore, naturally, a heat dissipating measure on heat generating articles like such products is necessary.
Here, the heat dissipating measure refers to a means for transporting and releasing thermal energy to the outside (a low temperature region) from a heat source (a high temperature region) inside a product by combining heat transfer means of heat conduction, convection, and heat radiation.
Conventional heat dissipation measures focus mainly on heat conduction and convection, and, for example, methods are used in which thermal energy derived from a heat source is allowed to conduct through the interior of a heat dissipating plate made of aluminum or copper, and thermal energy that has reached its surface is forcibly convected by a cooling fan or the like. However, due to the higher performance and smaller size of products, it is becoming difficult to secure space for installation of a physical heat dissipating means (such as a heat dissipating plate or a cooling fan) inside the products. Moreover, as products are smaller and denser, there is a trend toward using completely sealed housings to avoid an influence of fine refuse or dust, and in this case, a heat dissipating effect by convection cannot be expected. In addition, the sizes and the forms of heat dissipating plates and cooling fans are in many cases restricted from the design and economy viewpoints.
Accordingly, to date, various heat dissipating coating materials have been proposed from the viewpoint of heat radiation. Here, heat dissipating coating materials generally refer to coating materials in which inorganic particles that absorb and radiate thermal energy are blended with a binder resin that has adhesion to a substrate, and because such coating materials can have a large area, can be used irrespective of the shape of heat generating articles (articles that dissipate heat), and are easily applied, the use of such coating materials are increasingly favored in electric apparatuses, electronic apparatuses, optical apparatuses, and the like.
However, conventional heat dissipating coating materials are in many cases designed to bring the integral emissivity of a coating film close to the emissivity of a black body (=1) in order to radiate thermal energy over the entire wave length region or over the broadest possible wavelength region. For example, Patent literature 1 discloses, as a coating material that shows infrared heat dissipation efficiency close to that of a black body in the entire infrared wavelength region, a heat dissipating coating composition in which particles obtained by mixing and sintering a powder of three or more transition element oxides having mutually different wavelength regions and pulverizing the powder are contained. However, theoretically, infrared radiation (electromagnetic waves) radiated from a heat generating article does not release a constant amount of energy over the entire wavelength, but has an energy density distribution dependent on the wavelength specified by the temperature, and heat dissipating coating materials such as those showing infrared radiation efficiency close to that of a black body over the entire infrared wavelength can also absorb electromagnetic waves from outside. Therefore, such a heat dissipating coating composition is not suitable for products that need heat dissipating measures only in a specific temperature region.
Accordingly, for example, Patent Literature 2 proposes, as a coating material that can increase emission of heat in a temperature region of no higher than about 150° C. required in housings, heat dissipating plates, and the like of, for example, household electric appliances, a heat dissipating coating composition in which at least carbon black, which is a pigment having a thermal emissivity of 60% or greater at a wavelength of 6 μm, and titania, which is a pigment having a thermal emissivity of 60% or greater at a wavelength of 12 μm, are blended with various binder resins. With a focus on the point that housings and heat dissipating plates of household electric appliances and the like emit heat (electromagnetic waves) having a radiant energy density peak in a wavelength region of about 8 to about 10 μm in accordance with the so-called Planck's law of distribution, this coating material is a combination of two pigments that mutually complement radiation characteristics in this wavelength region.